1. Field of the Invention
The present invention relates to apparatus and methods for imposing an artificial phase shift upon the counterpropagating beam pair of a Sagnac interferometer. More particularly, this invention pertains to the deterministic sequences for effecting such artificial phase shifts to avoid undesired crosstalk and thereby overcome bias errors.
2. Description of the Prior Art
The Sagnac interferometer is an instrument for determining rotation by measurement of the nonreciprocal phase difference generated between a pair of counterpropagating light beams. This instrument generally comprises a light source such as a laser, an optical waveguide consisting of several mirrors or a plurality of turns of optical fiber, a beamsplitter/combiner, a detector and a signal processor.
In an interferometer, the waves coming out of the beamsplitter counterpropagate along a single optical path. The optical waveguide is "reciprocal"; that is, any distortion of the optical path affects the counterpropagating beams similarly although they do not necessarily experience such perturbation at the same time or in the same direction. Time-varying perturbations may be observed where the time interval is comparable to the propagation time of the light around the optical waveguide whereas "nonreciprocal" perturbations affect the counterpropagating beams differently and according to the direction of propagation. Such nonreciprocal perturbations are occasioned by physical effects that disrupt the symmetry of the optical medium in which the two waves propagate.
Two of the nonreciprocal effects are quite well known. The Faraday, or collinear magneto-optic effect, occurs when a magnetic field creates a preferential spin orientation of the electrons in an optical material whereas the Sagnac, or inertial relativistic effect, occurs when rotation of the interferometer with respect to an inertial frame breaks the symmetry of propagation time. The latter effect is employed as the principle of operation of a fiber gyroscope.
The measured or detected output of a gyroscope is a "combined" beam (i.e., a composite beam formed of the two counterpropagating beams after one complete traverse of the gyroscope loop.) The rotation rate about the sensitive axis is proportional to the phase shift that occurs between the counterpropagating beams. Accordingly, accurate phase shift measurement is essential.
FIG. 1 is a graph that illustrates the relationship between the intensity of the combined (output) beam and the phase difference between the counterpropagating composite beams. The fringe pattern as shown consists of two elements, a d.c. component and a component that is proportional to the cosine of the phase difference between the beams. Such phase difference provides a measure of the nonreciprocal perturbation due, for example, to rotation.
As a consequence of the shape of the fringe pattern, when small phase differences are to be measured (e.g. low rotation rates), the intensity of the combined beam is relatively insensitive to phase difference as the phase difference is then close to a maximum of the fringe pattern. Further, mere intensity of the composite beam does not indicate the sense or direction of rotation.
For the foregoing reasons, an artificially biased phase difference is commonly superimposed upon the counterpropagating beams. The biasing of the phase shift, also known as a "nonreciprocal null-shift", enhances the sensitivity of the intensity measurement to phase difference. A maximum degree of sensitivity is achieved by shifting the operating point of the gyroscope to .+-..pi./2. Furthermore, by alternating the bias between +.pi./2 and -.pi./2, two different operating points are observed. This enables the system to determine the sign of the phase difference and, thus, the direction of rotation.
In addition to phase modulation, the processing of the interferometer output commonly employs "phase-nulling" which introduces an additional phase shift through a negative feedback mechanism to compensate for that due to the non-reciprocal (Sagnac) effect. Commonly, the negative feedback generates a phase ramp whose slope is proportional to the rate of change of the measured phase difference. In actual practice, a ramp whose height varies between zero and 2.pi. radians is employed as the nulling phase shift cannot be increased indefinitely due to voltage constraints.
In the past, phase nulling has employed both analog and digital techniques. In analog phase nulling, a sawtooth waveform whose slope is proportional to the phase difference and whose peak-to-peak amplitude is equal to 2.pi. radians is combined with the above-described phase modulation (null shift) signal to drive the electro-optic phase modulator located within the gyroscope coil. The analog method is limited insofar as the scale factor of the phase modulation command differs from that utilized for the negative feedback. Furthermore, it is quite difficult to detect the 2.pi. radians peak-to-peak amplitude in the analog method described supra.
U.S. Pat. Ser. No. 4,705,399 of Graindorge, Ardity and Lefevre discloses a digitally-based arrangement that overcomes a number of the shortcomings of the analog sawtooth technique. In the patented system, a "stairstep" waveform replaces the sawtooth. The height of each step, .DELTA..phi..sub.0, is equal to the measured phase difference while the width or duration of each, t.sub.0, is the group delay time of the optical coil. FIG. 2 is a graph of a portion of such a feedback ramp signal. On the average, the slope of the ramp is equivalent to the measured nonreciprocal phase difference per unit of time. Two waves whose respective delay is equal to the group delay of the gyro loop are always on two consecutive steps and their phases differ by .DELTA..phi..sub.0. This method is compatible with digital signal processing and enjoys many resulting advantages. Additionally, the phase modulation may be directly added to the digital ramp through the synchronization offered by a digital signal processor.
The (combined) signal ultimately controls the phase modulator that is positioned near one end of the optical fiber coil. This device may comprise an electro-optic crystal whose index of refraction is responsive to an applied voltage or a piezoelectric structure arranged to exert pressure upon the optical fiber in response to an applied voltage. The pressure, in turn, affects the length of the somewhat-compressed optical fiber.
Many applications, including navigation, require rotation and position information with respect to the three orthogonal space axes. Accordingly, a triad of interferometers may be required, one for sensing rotation about each of the rotation axes. In the past, systems of this type have employed three independent interferometers. That is, each interferometer has utilized a single dedicated source of optical energy and a single photodetector. As a result, a total of three sources of optical energy and three photodetectors have been required. The use of multiple sources of optical energy and photodetectors adds significantly to the weight, power consumption, heat dissipation and cost of an overall navigation system. Also, a control circuit is required for each source, further adding to size, heat and power consumption problems. One solution to the element multiplicity problem has been proposed in pending U.S. patent application Ser. No. 705,762 of Goldner covering "Triaxial Fiber Optic Sagnac Interferometer With Single Source and Detector." An arrangement that eliminates the need for either multiple sources or detectors is taught. However, the modulation technique utilized requires the alternating "blanking" of two of three gyros at all times which limits the sensitivity, and hence accuracy, of the triax.
Both single axis and multiple axis gyroscope configurations are subject to bias errors arising from crosstalk between the gyro output signal and elements of the system. For example, parasitic signals from the output digital-to-analog converter and from the driver amplifier of the gyro control loop can couple into the synchronous demodulator input. In addition, in a triaxial arrangement, such cross-coupling can take place between the digital-to-analog converters and driver amplifiers of one axis-measuring gyroscope and the synchronous demodulators of the gyros for measuring the other axes. Furthermore, in the event that a triaxial arrangement is simplified to operate with a single detector, the composite output may be subject to crosstalk between the outputs of the other axes.
Spahlinger has proposed a solution to the crosstalk problem in pending U.S. patent application Ser. No. 652,422 covering "Fiber Optic Sagnac Interferometer With Digital Phase Ramp Resetting." This method relies upon random or pseudo-random based demodulation sequences to obtain mean-zero bias errors. Unfortunately, random-based methods are inherently "global". Accordingly, a relatively-large system bandwidth is required to overcome sometimes-misleading "local" data.